[20] Nimmo, F. ; Ermakov, A. ; Kang, W. ; Rovira-Navarro, M. ; Tobie, G. ; Van Hoolst, T.

published in Space Science Reviews, 222 issue 50, pp. 1-31 (2026)

Abstract: We review the key observations and theories relevant to the internal structure and dynam- ics of the Galilean satellites. Key observations include: the bulk densities and degree-two gravity coefficients of the moons; the presence of conductive subsurface layers, as inferred from magnetic induction; and the surface compositions. All the moons, with the possible exception of Callisto, appear to be differentiated (denser components have separated from lighter components). Ganymede and Io have iron cores; Europa may have one. The outer three moons all likely possess subsurface oceans; for Europa the ocean overlies rock, while for the other two it overlies higher-pressure ice phases. Io is partially molten but does not possess a shallow magma ocean. Tidal heating is the dominant energy source at Europa and Io, and may have affected Ganymede’s long-term evolution. The dynamics of the subsurface oceans are of considerable theoretical interest but are only weakly tied to current or likely future observations. We identify seven outstanding questions regarding internal structures, some of which will be answered by the forthcoming JUICE, Europa Clipper and Tianwen-4 missions.

Funding: PRODEX PlanetInterior/PRODEX PlanetInterior/PRODEX PlanetInterior